The present invention relates to the field of aviation, and more particularly to the air inlets of nacelles of turbine engines, i.e. airplane turboprops and turbojets.
In conventional manner, airplane engines are housed in nacelles situated at various locations on an airplane, in or under a wing, or indeed on the rear fuselage, for example, where such a nacelle has an air inlet oriented along its axis, with a generally annular leading edge at the front, and presenting a streamlined profile for guiding the air sucked in by the engine.
The sounds emitted by such engines originate mainly from the jet leaving the nozzle and also from the noise of the fan resulting from its blades rotating and moving past the fan casing.
At transonic speeds, where Mach numbers for the flow over the blades are locally greater than 1, shock waves form at the leading edges of the blades and propagate along spiral paths upstream from the inlet with decreasing amplitude, thereby creating noise with harmonics that are multiples of the blade pass frequency.
In order to limit the effects of the noise emitted forwards from the engine towards the ground, which noise is thus particularly audible during takeoff and landing operations, proposals have been made to use nacelles having an inlet profile of chamfered shape. The inlet plane of the engine thus slopes rearwards as a result of the bottom portion of the air inlet being lengthened. Nevertheless, such lengthening counters current constraints associated with weight, which tend on the contrary to shorten air inlets as much as possible.
Another solution, known in particular for reducing shock-related noise, is illustrated by application EP 1 783 346, which proposes modifying the fan casing by making grooves or splices therein so as to prevent shock waves from propagating in the air inlet. Unfortunately, those grooves lead to significant degradation of the acoustic performance of the inlet with other types of noise, and they also have a negative influence on the performance of the engine while cruising.